Glossary of GIS Terminology Part 4

Quadratic polynomial - One in which the highest degree of terms is 2.Quadtree - A data structure used to reduce the storage requirements of a raster by coding contiguous homogenous areas singly. A raster of 2n by 2n cells is recursively divided into four equal squares. Subdivision continues in each square until the square is homogeneous or subdivision is no longer possible.
Qualitative - In mapping, qualitative refers to feature labels that are not quantitative. For example, a "10" representing the land use category residential.

Quantization - Dividing a continuous range into a finite set of discrete values called quantization levels. If too few quantization levels are used, false contours may appear in an image. Quantization levels are often referred to as gray levels, but the concept can also apply to color images.
Raster algebra - Manipulations and functions that operate on raster objects cell by cell. Any raster object can be used as a variable, or operand, in a raster algebraic expression. A combination of operands is assembled in an equation and then a raster object is assigned to each operand. The result of the operation is stored in a new raster object.
Raster cell - One value in a raster defined by its row and column position. A raster cell value may be the elevation above sea level at one position in a survey site or the intensity of red radiation for a pixel in a video image. For convenience, a raster cell is usually thought of as square or rectangular, although many image collection devices actually measure circular or elliptical areas.
Raster display - A device for displaying information in the form of pixels on a CRT.
Raster map - A map encoded in the form of a regular array of cells.
Raster or raster object - A raster is a data structure (logically, a 2-dimensional array) that contains rows and columns of numbers of a single data type. Each number represents the value of some parameter (like elevation or red spectral intensity). Each number (or cell value) is often used to control the color and intensity of one pixel on a computer’s display screen. A complete computer image can be displayed from the values in a raster that has as many rows and columns as the computer has pixels for the screen.
Raster-to-vector - The process of converting an image made of cells into one described by lines and polygons.
Raster/vector conversion - The ability to convert data between vector and raster forms with grid cell size, position and orientation selected by user.
Rasterize reclassify - To convert data from vector format to cell format. In GIS, a type of data analysis entailing reassignment of values to existing spatial data.
Rectification - Removing geometric distortion from a raster or a vector object. Rectification is usually achieved by aligning raster features or vector coordinate positions with features in a base map or other coordinate reference framework. Rectification may be used to bring several distorted image segments into a common framework so they can be combined into a larger image.
Redundancy - The inclusion of data in a database that contribute little to the information content.
Region - An identified subset of the geographic database. Regions may be permanently defined and named subsets, such as a watershed, or interactively defined by the user through such methods as outlining an area on the display screen or by searching for an area of the database with common attribute values.
Registration - Geometrically aligning sets of image data so that corresponding features are coincident. (See also Co-registration)
Regression - A statistical technique that allows one to examine the relationship between two or more quantitative variables. This relationship is expressed in terms of the correlation between the variables, i.e., the degree of association, and a best fit trend line that express mathematically the character of the relationship.
Relief (as in shaded relief) - The variation in a raster object’s values that shows differences between a surface’s higher and lower parts in elevation and slope.
Remote sensing - The act of detection and/or identification of an object without having the sensor in direct contact with the object. Includes satellite imagery and aerial photography.
Rubber sheeting - Any process in which a raster is stretched differentially to match a new set of geometric constraints. This shape change could be defined by any one of many transformations such as changing a map projection, trilateration to change the absolute position of cells within a raster, fitting a polynomial to a surface, least squares movement of control cells, and so on.
Scale or map scale - The relationship that exists between a distance on a map and the corresponding distance on the Earth. It may be expressed as an equivalence, one inch equals 16 statute miles; as a fraction or ratio, 1:1,000,000; or as a bar graph subdivided to show the distance that each of its parts represents on the Earth.
Schema - Data schema. The organization in memory of data, and, in particular, the reference linkages among the data elements.
Seam - The junction in the area of overlap between raster objects combined by tiling or mosaicking.
Segment - (in the context of feature mapping) The area selected for analysis and measurement within a study site. The raster objects a user selects for processing may contain areas that the user does not want to include in his/her study. In segmentation the user uses a set of drawing tools to turn cells "on" and "off" for the subsequent processing steps. For instance, if one is interpreting airphotos to measure wetland features of duck habitat along a river floodplain, one can create a segment to restrict the feature mapping analysis to the floodplain, excluding other areas contained in the processing raster(s). If one does not define segments, the entire area covered by the raster object(s) is considered to be the processing segment. Segments may be larger than the display screen, may be discontinuous, and may have holes.
Sliver - A gap between two lines, created erroneously by a scanner and its raster-vector software.
Sliver polygon removal - To detect automatically the small sliver polygons that result from a polygon overlay operation when certain polygon lines on the two maps represent different versions of the same physical line.
Slope - A measure of how steeply a surface or line inclines. Slope is computed by dividing a line’s vertical rise or fall by the distance the line travels on the surface (the "rise over the run") - usually expressed as a percentage.
Small scale - A mapping scale which covers a relatively large area and has generalized labels. The term small refers to the fraction represented by the ratio of map distance to ground distance. For example, 1:500,000 (one map unit equals 500,000 ground units.)
SPANS - A GIS marketed by TYDAC Technologies Corporation. It has raster, vector, and quadtree data structures, and operates on PCs and workstations as a self-contained desktop GIS.
Spatial - Refers to phenomena distributed in two or three dimensional space and therefore having physical dimensions.
Spatial auto covariance - The characteristic that loci close together are more likely to have similar values than loci that are far apart.
Spatial query - A process of the extraction of cartographic data based on user specified windows, such as a circle, or other regular and irregular shapes.
Spatial resolution - Measure of the ability of an imaging system, such as LANDSAT, to separate the images of closely adjacent objects. Also, the smallest area identified as a separate mapping unit.
Spectral band or spectral region - A well-defined, continuous wavelength range in the spectrum of reflected or radiated electromagnetic energy. Red, green, and blue are all spectral regions within the portion of the spectrum that is visible to humans as light. Color-infrared images are composed of red, green, and a spectral region commonly called the photo infrared, which is not in the visible portion of the electromagnetic spectrum. (See also Electromagnetic spectrum, Color-infrared)
Spike - An overshoot line created erroneously by a scanner in its raster-vector software.
Spline - An interpolating polynomial for a set of coordinate points used to fit a curve that connects the points. (See also Bezier curve and Bicubic spline)
Spur - A false line segment that extends a short distance beyond a T-junction of two lines.
Stereo elevation - An elevation surface derived from stereo pairs of remote sensing imagery. For example the SPOT satellite can collect an off nadir image and then in a subsequent fly-over collect its counterpart to make a stereo pair. The stereo elevation process takes the 3D effect of that stereo pair to derive an accurate raster object of elevation values.
Stereo plotter - A device for extracting information about the elevation of the land surface from stereoscopic aerial photographs. The results are sets of (x, y, z) coordinates.
Supervised classification - A type of semiautomatic multi-spectral image interpretation in which the user supervises feature classification by setting up prototypes (collections of sample points) for each feature, class, or land cover to be mapped.
Suppress - The ability to exclude objects by attribute (the converse of selecting by attribute).
Surface fitting - Techniques that use 3-dimensional vector point data to create a raster object containing an elevation surface. Various methods such as polynomial and piecewise triangulation techniques can be used to fill the gaps between the vector points and derive the elevation values for the intermediate raster cells.
Surface mapping - A process to create a cartographic display of three-dimensional information in terms of two-dimensional or three-dimensional forms. This can be achieved through contour mapping of three-dimensional relief displays, or color, shading, hatching or other techniques.
Thematic map - A map related to a topic, theme or subject of discourse. Also called geographic, special purpose, distribution, parametric, or planimetric maps. Thematic maps emphasize a single topic such as vegetation, geology, or land ownership.
Thinning (Rasters) - To remove cells from wide line images in a raster object. When a scanner creates a raster object, the lines in the drawing typically are several cells wide in the result. Before automatic vectorization techniques can work on the data, the line images in the raster object must be thinned to make the line images just one cell wide.
Thinning (Vectors) - Reducing the number of coordinate pairs that describe a vector’s line and polygon elements. The process discards some of the coordinate pairs as straight line lengths replace curved or noisy segments in the original lines.
Three-dimensional (3-D) data - Volumetric data representing measurements in three dimensions, as angular or linear measures such as phi-lambda-kappa, latitude-longitude-elevation, etc.
TIFF - Tagged Image File Format. A series of standard color image file formats adopted by Microsoft, Aldus, and others to transfer images between different software packages.
TIGER files - Topologically Integrated Geographic Encoding and Referencing files compiled and distributed by the U.S. Census Bureau.
Tile - 1) Raster object image segments that are assembled and trimmed in the tiling process. 2) A way of arranging open windows so that no windows overlap but all windows are visible. Each window takes up a portion of the screen.
Tiling - Assembling large images from smaller segments that have common angular orientations, cell sizes, and map projections. The process is similar to that of assembling floor tiles, except that the raster tiles may have areas of overlap. (See also Mosaics)
TM - Thematic Mapper. A sensing device on the LANDSAT satellite that scans and stores 7 individual images in spectral bands ranging from the blue wavelengths up to those in the thermal infrared.
Topographic map, or topo map - A map that uses colors and symbolic patterns to represent the general surface features of the earth, such as grassland, forest, marsh, agricultural, urban, and barren rock.
Topography - The features of the actual surface of the Earth, considered collectively according to their form (grassland, cultivated, desert, forest, swamp, etc.). A single feature, such a one mountain or one valley, is called a topographic feature.
Topological codes (Topological relationships) - Codes that define the locations of data elements in space with respect to one another, but without reference to actual distances. Topological codes can be used to specify such relationships as point connectivity’s, grid cell contiguities, networks, polygon boundary segment chains, and area adjacencies. For a label to be topologically related to a graphic entity, an explicit logical connection between label and entity must be contained in the data record.
Topological overlay - The intersection of two or more topologically coded data sets that produces one data set that is uniformly topologically coded with respect to graphic entities and to attribute data.
Topology or vector topology - A description of the relationship between node, line, and polygon elements in a vector object.
Trace - To create a vector line element by manually or interactively tracing over line images in a raster object.
Transformed Vegetation Index or TVI - A commonly used vegetation index derived from images of certain spectral bands. The TVI is equal to the square root of the quotient of the photo-infrared minus the red band, and the photo-infrared plus the red band.
Transformation - Mathematical expressions used to convert coordinate data within one frame of reference to coordinate data in another frame of reference. It is used for a variety of applications including changing from one map projection to another map projection, or converting from one set of coordinates captured on a digitizer to UTM ground coordinates.
Trend surface analysis - The simplest way to describe gradual long-range variations is to model them by polynomial regression. The idea is to fit a polynomial line or surface, depending on whether the data are in one or two dimensions, by least squares through the data points. It is assumed that the spatial coordinates (x, y) and their powers and products are the independent variables, and that z, the property of interest, is the dependent variable.
True scale - At large sizes, every map projection distorts the scale of distance, especially towards the edges. The location of the true scale of a projection identifies the position where map measurements correctly correspond to actual surface distances.
Universal Transverse Mercator map projection - (UTM) A system of plane coordinates based upon 60 north-south trending zones, each 6 degrees of longitude wide, that circle the globe.
Unsupervised classification, also automatic interpretation - A multispectral image interpretation process (like K-means) that statistically clusters cells into similar collections. When the classification is complete, the user identifies and labels the ground features or conditions that the clusters represent.
Utility mapping - A special class of GIS applications for managing information about public utilities such as water pipes, sewerage, telephone, electricity, and gas network.
Vector - A data structure for representing point and line data by means of 2- or 3-dimensional geometric (Cartesian x, y or x, y, z) coordinates. In connection with GIS and computer graphics, vector can refer to a set of line segments joined end to end to make a curved path in space.
Vector elements (Vector data) - A vector object is made up of three different types of elements - 1) nodes, which are single sets of coordinates which define a point (such as a spring); 2) lines, which are curvilinear strings of coordinates which define a curved line (such as a stream); and 3) polygons, which are collections of lines which inscribe an area (such as a lake).
Vectorize - A general term for any technique that converts raster data into vector data.
Vegetation index - The output from standard manipulations of multispectral image raster objects. The system processes the input spectral information and creates output raster objects whose cell values represent the site’s biophysical properties - amount of vegetation, leaf area, greenness, brightness, and wetness.
Vertex - The endpoint or intersection of lines or arcs.
Warping - Any process in which an object is stretched separately so as to change its internal geometry. This shape change could be defined by any one of many transformations, such as changing a map projection, trilateration to change the absolute position of specified nodes in a vector object, fitting a polynomial to a surface, least squares movement of control nodes, and so on.
Watershed -  It is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another water body, such as a riverlakereservoirestuarywetlandsea, or ocean.
UPDATED:  July 9, 2017